remove support for rustc_safe_intrinsic attribute; use rustc_intrinsic functions instead

src/doc/unstable-book/src/language-features/intrinsics.md

@@ -18,7 +18,7 @@ All intrinsic fallback bodies are automatically made cross-crate inlineable (lik
 by the codegen backend, but not the MIR inliner.
 
 ```rust
-#![feature(rustc_attrs)]
+#![feature(intrinsics)]
 #![allow(internal_features)]
 
 #[rustc_intrinsic]
@@ -28,7 +28,7 @@ const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {}
 Since these are just regular functions, it is perfectly ok to create the intrinsic twice:
 
 ```rust
-#![feature(rustc_attrs)]
+#![feature(intrinsics)]
 #![allow(internal_features)]
 
 #[rustc_intrinsic]

src/tools/miri/tests/fail/intrinsic_fallback_is_spec.rs

@@ -1,4 +1,4 @@
-#![feature(rustc_attrs)]
+#![feature(intrinsics, rustc_attrs)]
 
 #[rustc_intrinsic]
 #[rustc_nounwind]